1. Field of the Invention
This invention relates to glasses and fibers for laser applications and more specifically to Thulium-doped heavy metal oxide glasses for near 2 μm lasers with high quantum efficiency.
2. Description of the Related Art
2 μm lasers are highly transmissive in air, relatively eye-safe, and highly absorptive in many natural substances such as organic tissue and water. Applications such as LIDAR, range finder, materials processing, and biomedical require low cost, robust and efficient 2 μm, e.g. typically 1.7-2.1 μm, lasers. As used herein the term “2 μm” laser refers to laser emission across the range of 1.7-2.1 μm.
Most 2 μm Thulium doped lasers use a crystalline host material. Crystalline materials exhibit good mechanical strength and chemical durability and through a phenomenon known as “cross-relaxation” exhibit high quantum efficiency. Unfortunately crystalline materials are expensive and cannot be pulled into fibers, hence can only be used for free space solid-state lasers.
Glass hosts, namely silica and fluoride glass, which can be pulled into fiber, have been developed for 2 μm fiber lasers. Unfortunately, neither glass is a good host for Thulium doped laser applications. Silica glass has high phonon energy, approximately 1100 cm−1, and can only support limited doping concentrations, and thus exhibits poor quantum efficiency. Although fluoride exhibits low phonon energy of about 700 cm−1 and accept high rare-earth doping concentrations, fluoride glass is physically weak and chemically not durable, which limits the output power of the laser and seriously restricts practical application.